Project Summary Pegloticase is highly efficacious therapy for chronic refractory gout patients (n = > 400K in the US alone). It decreases serum urate (sUA) levels to often undetectable levels and reduces tophi burden. However,its long- term real world effectiveness is severely limited due to its immunogenicity caused by anti-pegloticase antibody formation. REducing Immunogenicity to PEgloticase (RECIPE) is a Phase II, double-blind, placebo controlled, proof-of-concept trial in 32 subjects initiating pegloticase for treatment of chronic refractory gout. RECIPE will investigate the preliminary efficacy and safety of using immune modulating therapy with mycophenolate mofetil (MMF) to prevent immunogenicity conferred by pegloticase. Subjects will be randomized 3:1 to receive MMF vs. placebo in addition to everyone receiving pegloticase. The co-primary aims of the RECIPE trial are to 1) determine if a 12 week course of immune modulating therapy with daily MMF can safely and significantly attenuate immunogenicity to pegloticase as determined by the proportion of participants achieving and maintaining an sUA less than or equal to 6 mg/dL through 12 weeks, compared to concurrent controls, and 2) to assess the incidence and types of adverse events and infusion reaction. After 12 weeks of co- administration, all participants will continue on pegloticase for an additional 12 weeks without combination MMF immune modulating therapy to evaluate the longer term benefits (durability) and safety of this approach. The secondary aims are to: 1) Determine the 6 month durability of immune modulation after discontinuation of the short course of MMF by: a) assessing the absolute change in sUA from baseline to Week 24, and Week 12 to Week 24 and b) determining the proportion of participants with sUA ? 6 mg/dL through 24 weeks, and Week 12 to Week 24; 2) Identify and characterize the pegloticase immune response by immunoglobulin isotypes (IgG and IgM), specificities, and antibody titer; and 3) Examine patient reported outcomes (PROs) using the NIH supported Patient Reported Outcomes Measurement Information System (PROMIS) and Gout Impact Scale (GIS) instruments. The University of Alabama at Birmingham (UAB) and the University of Michigan (UM), two large academic gout and immunology research centers, which in aggregate see nearly 10,000 gout patients annually, will serve as the two study sites and are very well-positioned to address the clinical and immunologic questions posed. Our collaborative expertise, complemented by the leveraged resources of the NIAMS supported P50 UAB Center of Research Translation (CoRT) in Gout and Hyperuricemia will be used to execute this novel proof-of-concept trial with a long-term objective of improving gout treatment and quality of life and better understanding the mechanism of pegloticase immunogenicity.